X-ray imaging technology provides a non-invasive technique for visualizing the internal structure of an object of interest by exposing the object to high energy electromagnetic radiation (i.e., X-rays). X-rays emitted from a radiation source pass through the object and are absorbed at varying levels by the internal structures of the object. As a result, X-ray radiation exiting the object is attenuated according to the various absorption characteristics of the materials which the X-rays encounter.
The absorption characteristics of the object of interest may be captured by placing the object between a high energy electromagnetic radiation source and an image recording medium. As radiation from the source passes through the object, the radiation impinges on the image recording medium with an intensity related to the attenuation caused by the different absorption characteristics of the object. The impinging radiation causes a change in the image recording medium that is proportional to the radiation intensity, thereby storing information about the internal structure of the object. The image recording medium may then be processed to recover the stored information by, for instance, converting it into digital form. Common types of image recording media include sheet film, phosphor media, and the like.
Phosphor plate technology has emerged as a valuable image recording media for computed radiography (CR). When electromagnetic radiation, such as X-ray radiation, impinges on a phosphor plate, the radiation interacts with the phosphor lattice of the plate. The phosphors in the plate store energy proportional to the intensity of the impinging radiation. This energy can later be released by scanning the plate with a laser to excite the phosphors in the plate (i.e., by causing the phosphors to fluoresce). The excited phosphors release radiation that can be detected, quantified and stored as values representing pixels in an image.
Some imaging procedures require exposure to radiation of relatively large-dimensioned objects, referred to generally as elongate objects. Elongate objects typically have at least one dimension larger than standard large-sized commercially available phosphor plates. For example, in various medical imaging procedures, elongate structures involved in full-leg or spine examinations may need relatively long portions of the body to be imaged at once. Such procedures are often referred to as “long bone” imaging and typically require specialized equipment to accommodate the elongate properties of the anatomy being imaged. In addition, imaging of elongate objects in the industrial setting is often desirable. For example, non-destructive imaging to test for structural integrity of structural supports such as pillars or load-bearing walls and/or imaging of plumbing such as pipes may include elongate objects that extend further than the capacity of a single conventional phosphor plate.
Conventional size radiographic image acquisition units are limited in size and unsuitable for imaging elongated body regions such as the full spine or the leg. When it is necessary to obtain a radiographic image of a full spine or leg, several approaches have been used. If film/screen technology is used, either an extra long, non-standard radiographic film is used. For example, see U.S. Pat. No. 5,130,541 (Kawai). Alternatively, as disclosed in U.S. Pat. No. 3,774,045 (Trott) and U.S. Pat. No. 3,725,703 (Bucky), a cassette or cart is provided for holding a plurality of overlapping conventional sized film cassettes or packs.
A further alternative involves placing two or more standard sized CR phosphor plates (e.g., 35×43 cm phosphor plates) lengthwise adjacent to one another in a specialized cassette. The combined lengths of the multiple plates may be sufficient to image an elongate object having a dimension greater than the longest dimension of any single phosphor plate. The term “cassette” refers generally to any of various casings, cartridges or containers adapted to hold other material, and more particularly, adapted to hold one or more image recording media (e.g., one or more phosphor plates) to protect against damage from direct handling, contact or exposure. For example, a cassette may be formed as a rigid encasement providing a shell that can withstand the weight of a patient, rough handling, accidental falls, etc. A cassette typically includes some form of opening that permits insertion and extraction of the image recording media into and out of the cassette.
Cassettes employing two or more storage phosphor plates/screens have been disclosed. For example, see commonly assigned U.S. Pat. No. 6,852,987 (Steklenski), U.S. Pat. No. 6,696,691 (Foos), and U.S. Pat. No. 6,744,062 (Brahm). See also U.S. Pat. No. 6,843,598 (Minnigh), U.S. Pat. No. 6,273,606 (Dewaele), EP 1 312 977 (Delaby), JP 2000-241920 (Sasada), JP 2002-202571 (Nakajo), JP 2000-267210 (Sasada), JP 2000-250153 (Sasada), and JP 2000-258861 (Sasada).
FIG. 1 illustrates an exemplary elongate body imaging apparatus. The apparatus includes a cassette 100 having a width X and a length Y. Cassette 100 is adapted to hold a pair of phosphor plates 150a and 150b, each having a width x and a length y. In general, the length Y is chosen to accommodate a desired long bone imaging procedure, for example, an examination of an entire leg or spinal column. The lengthy is typically equal to the length of conventional phosphor plates produced by major vendors of imaging plates, and is insufficient, on its own, to capture complete image information for the entire elongate object.
Phosphor plates 150a and 150b are placed in cassette 100 in an overlapping relationship to form an overlap region 155 having a width u. Together, the dimensions of plates 150a and 150b (i.e., 2y-u) is sufficient to capture image information of the entire elongate object.
Cassette 100 may include reference or fiducial marks (not shown) that cast a “shadow” on portions of the plates, for example, in overlap region 155. The reference marks may be any generally X-ray resistant material arranged in a known pattern or relationship to one another that, when exposed to X-ray radiation, will cast a distinguishable shadow on the phosphor plates. For example, the reference marks may be a plurality of parallel and perpendicular lines that, when exposed, imprint a grid pattern on the phosphor plates. The reference marks provide fiducial information that may be referenced to correct for alignment errors and as a guide to facilitate image stitching, as discussed in further detail below.
The above described action of overlapping multiple plates in a cassette may be repeated to achieve coverage of elongate objects of any desired dimension. For example, a cassette may be adapted to hold a third plate having an overlapping relationship with the bottom edge of plate 150b. Any number of plates may be positioned together to arrive at a length sufficient to obtain image information from a desired elongate object.
After the cassette is exposed to radiation in the presence of the elongate object, phosphor plates 150a and 150b are removed from the cassette and are loaded individually and successively into a CR image reader adapted to acquire image information stored on the standard sized plates. Accordingly, the CR image reader obtains a first partial image from phosphor plate 150a and a second partial image from phosphor plate 150b. 
However, the separate partial images make diagnostic and/or examination of the elongate object relatively difficult. Thus, the CR image reader may include image processing techniques adapted to stitch the partial images together using alignment information provided by the reference marks imprinted on the partial images to form a single image of the elongate object. Some stitching methods have been disclosed, for example, see commonly assigned U.S. Pat. No. 6,895,106 (Wang).